PRELIMINARY STUDY FOR DETERMINATION OF DISTAL DOSE EDGE BY MEASURING 90˚PROMPT GAMMAS WITH ARRAY-TYPE PROMPT GAMMA DETECTION SYSTEM

Abstract

Purpose: The purpose of this experiment was to verify that it is possible to determine the ‘distal dose edge’ of a proton beam (where the dose in the medium decreases to 80% of its peak dose after the Bragg peak or SOBP) by measuring prompt gammas with an array-type prompt gamma detection system. Methods and Materials: A 45-MeV proton beam was delivered to a water phantom (20 cm x 20 cm x 40 cm), and then the prompt gammas, generated in the passage of the proton beam in the water phantom, were directly measured with an array-type prompt gamma detection system, which is composed of (1) a collimator with multiple collimation slits, (2) an array of CsI(Tl) scintillators and photodiodes, and (3) lead shielding. The distal dose edge was measured separately with a plane-parallel ionization chamber and, then, compared with the distribution of the prompt gammas. To evaluate the contribution of the background gammas, the experiment was also simulated with a Monte Carlo particle transport code, MCNPX. Results: The distribution of the prompt gammas, measured by the array-type prompt gamma detection system, sharply decreased at the distal dose edge, within the range of a few millimeters. The effect of the background gammas was not very significant at least for the 45-MeV proton beam case. Conclusions: The results were very encouraging, and demonstrated that the distal dose edge of the proton beam in the patient can be determined by directly measuring, during treatment, the distribution of the prompt gammas with an array-type prompt gamma detection system.